Natural vegetation is an indicator of the climate. But they are interrelated in such a way that one influences the other to a great extent.
Though the influence of the natural vegetation is confined to the area covered by it and its influence is proportional to its density, it also influences the surrounding area.
In the forests the temperature is actually less than that of the open areas. Besides, in the forested areas, the range of temperature is comparatively less.
According to Critchfield, the annual average temperature of the forested areas in the plains of temperate zone is generally 0.6°C less than that of the open grounds. However, at an altitude of 100 meters the difference in temperature is about 1°C.
During the summer months the difference in temperature between the forested areas and the open ground is a little more (1.4° C) than that during the winter season. But in the tropical forests because of the shade of the trees the temperatures are usually much lower.
In the dense forests the upper parts of the trees do not allow the ground under them to be much warmer during the day. But at night the eradiation from the upper parts of the trees takes place at a faster rate than that from the ground which takes relatively longer time to cool.
In the same way, because of the dense canopy of the tree tops during summer and spring seasons the forest-covered ground takes longer time to get warmer than during winter and autumn.
In the forests the undergrowth in the form of small grasses or bushes is scanty. This is so because about 90 percent of the insolation falling on the tree tops is absorbed there and very small amount of solar radiation reaches the ground.
So in the absence of necessary heat the undergrowth in the densely forested areas is rather negligible. As regards rainfall, the amount of rain in the forests is much more than what it is in the open areas.
Depending on the density of tree tops a certain percentage of rainfall gets evaporated before reaching the ground.
Another notable point about the contribution of forest trees to the ground moisture is that the leaves of the forest trees withhold tiny water droplets produced by advection fog and release them gradually on the ground to provide additional moisture to it.
In comparison with the open areas the relative humidity in the forest areas is 3 to 10 percent more because of certain factors like relatively lower temperature, impediments in the wind speed and the continuing transpiration from the trees.
Where the ground is covered with natural vegetation, loss of moisture from the ground through evaporation is usually 50 to 70 percent less than that from the naked ground.
The wind blowing form the forested areas towards the open areas are helpful in checking the occurrence of frosts during the winter months because such a wind carries with it certain amount of dew and fog with it.
Forests act as barriers to the wind movement reducing its speed. This leads to reduction in transpiration and temperature and enhancement in the relative humidity. These facts must be taken care of by those related with forestry.
The knowledge of optimum climatic conditions in different stages of the growth of forest trees is essential for those engaged in the task of afforestation for timber, watershed management or for ensuring the supply of forest products which are many in number.
In silvicultural practices the interrelationship between forests and climate must be taken into account. Since forests need higher moisture than most other types of plants, it is but natural that forests are associated with humid climates.
However, even in humid regions the local differences in altitude, exposure to sun- shine, and slope have much influence on the distribution and rate of growth of forest trees. There are individual tree species which need optimum climatic conditions.
These conditions include the general atmospheric climate and the climatic conditions within the forest itself.
In a climax forest only one or a few more tree species dominate. In such forests other trees do not find favourable conditions for their growth because of shading by the existing forest.
There are certain tree species which need protection from direct sunlight in the early stage of their growth. However, when such species are mature enough they obstruct the growth and development of those species which in their early stage gave protection to them.
Scientific forestry is a very delicate subject. One has to take into account the microclimate of the forest for getting the best results in the form of higher yields. For this purpose, there is one method called the ‘logging method’.
This method involves burning and thinning with a view to get a convenient forest association. This method has to ensure sufficient light reaching the forest floor. In case the forest is very dense, the trees compete for sunlight, and in their attempt to do so they become very tall, straight and thin.
Therefore subsequent thinning makes room for more sunshine to reach the ground. Besides, thinning also provides more space for the growth of trees. However, too much of thinning is equally harmful to the standing trees exposing them to damage by strong winds.
Thus in scientific forestry or planting of an altogether new forest (afforestation) a correct knowledge of the climatic conditions ideal for the growth at all stages is the basic necessity for the most successful management for timber production, watershed protection and other primary and secondary forest products.